If there are problems with updating like you get or with the permissions then easiest is to download the full version and trash the currently installed version to do a clean install of the new version.

Download a new copy of the Firefox program and save the disk image (dmg) file to the desktop

• Firefox 15.0.x: http://www.mozilla.org/en-US/firefox/all.html

• Trash the current Firefox application (e.g. open the Applications folder in Finder and drag the Firefox application to the Trash) to do a clean (re-)install
• Install the new version that you have downloaded
• https://support.mozilla.org/kb/Installing+Firefox+on+Mac

Your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks and other personal data if you uninstall and (re)install Firefox.

• http://kb.mozillazine.org/Profile_folder_-_Firefox

I have same problem. I keep getting message that new update is available for 15.0.something and if I click the option for "more information about this update" all I get is taken to the Mozilla main webpage, which doesn't tell me anything more about this update.

If I go ahead and start to download... it will say it's updating and show the line with the stuff moving so something is happening... forever. Literally. Hours and hours.

When I went to delete and redownload as suggested above, I notice that Firefox is not in the applications area, it's in "Devices" ...  ?????

Please advise. I don't have time for this mishigas! (a good yiddish word, look it up, but it means what it sounds like it would mean).